In vehicles of today, a number of different safety systems are present. An example of such a safety system is the seat belt pre-tensioner. The main task of the seat belt pre-tensioner is, often by use of an electric motor, to tighten the seat belt in order to pull the occupant against the backrest of the vehicle seat, thereby reducing the risk of injury in the event of a collision. Further, the seat belt mechanism ensures that the applied seat belt rest firmly but comfortably against an occupant's body during normal driving circumstances. Reversible seat belt pre-tensioners are active safety systems, intended to be activated a number of times based on signals like the steering angle and the speed of the vehicle indicating dangerous driving situations and increased possibility of a crash. Such systems are often provided with a rotation sensor to monitor the seat belt webbing refraction speed.
When a seat belt is not in use, it is retracted or wound up properly around the belt reel or spool. After the occupant has un-latched the seat belt connector from the latch, a retractor retracts the webbing and the connector via, for example, a mechanical spring mechanism present in the refractor, until the webbing and the connector reach a fully refracted, stored position. The requirements on the retracting mechanism are to robustly, securely, and completely retract the webbing even if some interference or drag is encountered, i.e. the connector is leaning on the door sealing in the door opening, or in the case of environmental parameters like low temperature, or during any other circumstances interfering with the retracting procedure. Thus, to ensure proper functionality, such mechanisms usually apply a relatively large retraction or rewinding torque on the belt reel, resulting in a relatively high retraction speed.
A problem may arises at the end of the retraction procedure, namely that due to high refraction speed, the connector may strike against the vehicle interior trim and result in dents, marks, or other minor damage.
In is known to employ an electric motor used in a seat belt pre-pre-tensioner system to also retract the seat belt when it is not in use. In a known device, when release (unlatching) of the seat belt latch is detected, refraction of the seat belt starts with a constant magnitude of driving force applied by the motor to the seat belt spool. This system is an active system, totally depending on predetermined actions activated by predetermined steps and/or conditions. The seat belt retraction action is started by a signal, indicating release of the seat belt connector from the seat belt latch. Thereafter the electric motor starts the retraction of the seat belt with high speed, and then, when a specific point in time is reached, the speed of the electric motor is reduced.
A disadvantage of such an active pre-tensioning system is that it depends on signals generated by actions initiated by the user of the seat belt, and that the system always reacts in the same way when such signals are present. As is known from experience when it comes to the human mind, human beings are likely to not act exactly in the same way in one situation compared to another. Applied to the above described system, a problem may occur for example if a driver of a car releases the seat belt latch, then immediately changes her/his mind and wants to refasten the seat belt with the latch to drive away again. The motor-driven retraction action starts when the seat belt is released, and as a result the user may find him/herself pulling the belt against the force of the electric motor. A similar problem may occur if the user of a seat belt attempts to temporarily loosen the seat belt in order to, for example, reach something in the back seat or in the glove compartment, and simply wishes to refasten the seat belt directly after. Such situations may irritate the user, giving the user the impression of not being in control of his/her own seat belt, and may also cause damage to the electric motor or other parts of the retraction mechanism.